1. Field of the Invention
This invention relates to sliders for slide fasteners provided with discrete coupling elements and particularly to such a slider which has means of automatically locking the same against movement.
2. Related Art
A conventional slider of the automatic lock type is typically provided with a pull tab and a spring-actuated locking prong operatively associated therewith such that upward tilting of the pull tab lifts and releases the locking prong from engagement with the coupling elements of the slide fastener, and bringing the pull tab back in a position parallel with the slider body urges the locking prong down into the path of the coupling elements to lock the slider against any movement. Design considerations have been given such that the slider has on one hand a lock function in which the locking prong penetrates in between and locks adjacent coupling elements against movement under and up to a predetermined amount of pressure, and on the other hand a ratchet function in which with greater pressures the locking prong ascends and allows the coupling elements to move underneath and past the locking prong. The maximum allowable lock strength of the locking prong is defined by a critical pressure at which the coupling elements begin to shift out of position on the fastener or otherwise sustain damage. Certain structural features have been proposed, whereby the coupling elements when subjected to a pressure beyond the critical point are allowed to move, while being tilted, past and underneath and clear the locking prong without suffering physical damage. This device has a vertically extending locking surface and a canted cam surface contiguous thereto and includes a cavity in the bottom wall of the slider in confronting relation to the locking prong, the cavity being progressively reduced in depth toward the position of the diamond head to provide an upwardly slanted cam surface. Such device can perform the necessary lock and ratchet functions provided that a predetermined length of the locking surface and a predetermined angle of the canted cam surface are accurately maintained. However, the length of the locking surface is determined by the dimensional relations between a slider guide channel, a locking prong and coupling elements. The amount of ingress of the locking prong into the guide channel is variable considerably with dimensional errors resulting from aggregated tolerances in the finish of locking prong, guide channel and coupling elements or in the chamfering of coupling elements, should individual tolerances of these parts be small even below point mm. All these dimensional errors when added up would result in unduly strong lock function or conversely in greater ratchet function or reduced lock effect. This means that an extremely high degree of accuracy is required for the finished parts dimensions.